Improvement in surface properties of extrusions from Mg-Al-Zn based alloy machined chips

In order to recycle machined chips, hot extrusion tests were performed on magnesium alloys AZ31B and AZ91D. Two types of extrusion dies were used - one with an exit corner having a sharp rectangular edge (N-die) and the other with an exit corner having a round edge (R-die); the radius of the round edge was 1.5mm. In the extrusion tests using N-die, peculiar and wavy surface roughness that was almost perpendicular to the extrusion direction was formed in the extrusions obtained from machined chips of both the alloys. This roughness appears to be due to discontinuous metal flow caused by the basal plane slip along the exit corner of N-die. Therefore, the exit corner of R-die was essentially effective in improving the surface roughness of the extrusions because it led to smooth and continuous metal flow. The surface roughness tended to decrease with an increase in the extrusion ratio R from 19 to 100. Grain boundary sliding in the extrusion surface layer at the die exit appears to be a possible mechanism of another type of surface roughness, particularly in the extrusion tests performed with lower extrusion ratios, i.e., at lower strain rates.